1. Field of the Invention
The present invention is in the field of instruments used in connections with explorations or production of oil, gas, steam or other minerals in or from downhole formations. More particularly, the present invention is in the field of acoustic instruments used in connection with such downhole work. Still more particularly, the present invention relates to a sound isolator bar or component of acoustic instruments used in such downhole work.
2. Brief Description of the Prior Art
Several instruments utilized in downhole formation evaluation operate on acoustic principles. In essence, such instruments include a sound emitting transducer and one or more sound sensors positioned at predetermined distances from the transducer. Sound or acoustic waves emitted by the transducer travel through several media to the sensor or sensors. These media include any water, drilling mud or other liquid that surrounds the acoustic instrument in the downhole location and through any pipe, pipe casing and possible cement bonding around the casing, through the earth formation, and through the instrument itself. In order to obtain useful information for example about the cement bonding around the pipe casing, or the formation itself, it is necessary for the sensors to distinguish among the sound signals which have reached the sensor or sensors after having traveled through the several above-mentioned media. This is usually possible because sound travels at different speeds in different media. Nevertheless it was found in practice to be difficult to distinguish between sound waves that have traveled through the instrument housing itself and through metal, such as pipe casing, that surrounds the instrument. It is well understood in the art that sound that has traveled through the instrument housing itself carries no useful information about any media surrounding the instrument itself.
An important example of the use of acoustic instruments is in the field of logging instruments for evaluation of cement bonding to pipe casings in downhole formations. As is known in the art, a pipe casing in a downhole formation utilized for the production of oil, gas, steam, or other minerals is typically surrounded by a layer of cement that ideally should be tightly bonded to the metal casing. The integrity of the bond between the metal casing is of such importance that acoustic instruments have been developed in the art to measure the integrity or quality of the casing-metal-to-cement bond and to create a xe2x80x9clogxe2x80x9d of the corresponding data along the length of the pipe casing. More particularly, the acoustic instrument utilized for this cement logging purpose includes a sound emitting transducer and a plurality (usually two) sound sensors positioned at predetermined distances (usually at 3 and 5 feet) above the transducer. The instrument is lowered into the borehole on a wireline, centered within the casing by means that are usual for centering downhole instruments in a pipe and the transducer is activated by electrical energy supplied to it through the wireline. Sound waves (acoustic energy) generated by the transducer travel through several paths to the two sensors located in the instrument above the transducers. One eminent path of the sounds is through the liquid (water or drilling mud) that usually fills the pipe casing at this stage of the downhole operation or exploration, another paths is through the metal casing, still another through the formation, and yet another through the body of the instrument itself. The basic principle behind using acoustic energy to collect data on the integrity of cement bonding to the casing is that cement bonded tightly to the metal casing significantly attenuates the sound energy that is conducted through the pipe, much the same as a steel tube held tightly in a vise xe2x80x9cringsxe2x80x9d significantly less when struck by a hammer than a free standing steel tube. Thus, it is important for the instrumentation that receives data from the sensors to identify the acoustic energy that reaches the sensor(s) through the casing, and distinguish it from acoustic energy that has traveled to the sensor(s) through other routes.
Generally speaking sound waves (acoustic energy) travel through aqueous fluid at the speed of approximately 180 to 220 xcexcsec/foot, through steel at approximately 57 xcexcsec/foot and in the formation at the speed of approximately 45 to 200 xcexcsec/foot. Based on these different speeds an instrument that receives input from the sensors that measure the timing of the sound waves"" arrival as well as their intensity (amplitude) can usually differentiate on the basis of the timing of their arrivals (and other factors) among the sound waves that have traveled from the transducer to the sensor(s) through the liquid inside the casing, the steel pipe and the formation. However, as it was noted above, in this cement logging and also in other uses of acoustic instruments in downhole formations, the sound waves traveling through the metal body of the instrument itself are difficult to distinguish from the sound waves (acoustic energy) that reaches the sensor(s) through the metal casing. Moreover, in this cement logging application as well as in other downhole applications, the acoustic energy transmitted through the body of the instrument carries no useful information regarding the formation nor about the integrity of the cement bonding to the casing.
The prior art has coped with the just-described problem in various ways. One method of solution utilized in the prior art is to place a lead-filled pipe section as part of the body of the instrument, that is separating the sound emitting transducer of the instrument from the sensors by a lead-filled pipe section that acts as an xe2x80x9cisolator barxe2x80x9d. An important improvement over this method, still using a lead bonded steel tube as an isolator bar is described in U.S. Pat. No. 6,173,606. However, perhaps the longest known and still most frequently used way to prevent acoustic energy that has traveled through the instrument housing from interfering with acoustic energy that has traveled through other media (such as the pipe casing or the formation) is to provide an isolator bar that comprises a highly slotted steel body. Sound waves traveling through the highly slotted steel body must travel through multiple and extended paths whereby they arrive later than they would through an ordinary pipe section, and tend to cancel each other due to interference. This phenomenon or result is sometimes referred to as xe2x80x9cisolatingxe2x80x9d or xe2x80x9cattenuatingxe2x80x9d sound that has traveled through the instrument.
FIG. 3 of the appended drawings of this application shows an isolator bar of the prior art which includes a plurality of slots in its body. As is illustrated in this drawing, the isolator bars of the prior art for the purpose of attenuating sound traveling through the instrument body have always included slots which were perpendicular to (at 90xc2x0 angle) to the longitudinal axis of the instrument itself. In an isolator bar which is disposed in a nominally vertical position these slots appear horizontal, as shown in FIG. 3. The effect of these prior art slots on the structural strength of the isolator bars of the prior art must be understood in the context that an isolator bar typically is of several feet in length and has a relatively small diameter because it is part of an instrument that must fit within a pipe or in a borehole in a formation. The range of diameter of such instruments, including the isolator bar, is between 0.75 to 12 inches, with instruments being between 1 to 3 inches in diameter being more typical. It can be readily seen, and experience has shown that isolator bars with slots which are perpendicular to the longitudinal axis of a pipe of several feet in length and have the above-noted diameters are significantly weakened, particularly against bending forces. This is a serious disadvantage of the slotted sound isolator bars of the prior art. The present invention provides a slotted isolator bar that eliminates or significantly reduces the problem of structural weakening caused by the slots placed in the bar in accordance with the prior art.
It is an objective of the present invention to provide a slotted isolator bar used in conjunction with acoustic instruments in downhole formations, where the structural integrity and resistance of the isolator bar against bending forces is not significantly weakened.
It is another objective of the present invention to provide an acoustic logging instrument which has an isolator bar that meets the first above-noted objective.
It is still another objective of the present invention to provide an acoustic logging instrument in combination with a xcex3-ray tool, the combination including an isolator bar that meets the first above-noted objective.
The foregoing and other objectives and advantages are attained by an isolator bar which comprises a substantially cylindrical tube of metal of the type from which downhole instrument housing is usually made. A plurality of slots are placed in the tube at angles which are between 10 to 80xc2x0 angle relative to the longitudinal axis of the tube. The isolator bar is equipped with traditional mechanical means with which it is incorporated in an instrument, placed between an acoustic transducer and an acoustic sensor or sensors utilized in the instrument. The angled slots cause sound waves traveling through the isolator bar to travel through multiple and extended path to the acoustic sensor, without significantly weakening the structural strength of the isolator bar.